1.5v Alkaline battery principle and process

 

1.5v Alkaline battery is a secondary battery (rechargeable battery), which mainly relies on Li+ to embed and de-embed between two electrodes to work. With the continuous development of downstream industries such as energy vehicles, the production scale of lithium batteries is expanding.

 

This topic is divided into two parts, the upper and lower parts, of which the upper part focuses on the principle, formula and process flow of lithium batteries. The lower part explains the production and performance of lithium batteries. This article is the upper part of this topic.

 

I. Working principle

 

1. Positive electrode structure

 

LiCoO2+conductive agent+adhesive (PVDF)+current collector (aluminum foil)

 

2. Negative electrode structure

 

Graphite+conductive agent+thickener (CMC)+binder (SBR)+current collector (copper foil)

 

3. Working principle

 

3.1 Charging process

 

A power supply charges the battery. At this time, the electrons e on the positive electrode run from the external circuit to the negative electrode, and the positive lithium ions Li+ "jump" from the positive electrode into the electrolyte, "climb" through the winding holes on the diaphragm, "swim" to the negative electrode, and combine with the electrons that have already run over. At this time:

 

The reaction occurring at the positive electrode is:

 

The reaction occurring at the negative electrode is:

 

3.2 Battery discharge process

 

There are constant current discharge and constant resistance discharge. Constant current discharge is actually adding a variable resistor that can change with the voltage in the external circuit. The essence of constant resistance discharge is to add a resistor to the positive and negative electrodes of the battery to allow electrons to pass through. It can be seen from this that as long as the electrons on the negative electrode cannot run from the negative electrode to the positive electrode, the battery will not discharge. Electrons and Li+ move at the same time, in the same direction but different paths. During discharge, electrons run from the negative electrode through the electronic conductor to the positive electrode, and lithium ions Li+ "jump" into the electrolyte from the negative electrode, "climb" through the winding holes on the diaphragm, "swim" to the positive electrode, and combine with the electrons that have already run over.

 

3.3 Charge and discharge characteristics

 

The positive electrode of the battery cell uses LiCoO2, LiNiO2, and LiMn2O2. Among them, LiCoO2 is a crystal with a very stable layer structure, but when x Li ions are taken away from LiCoO2, its structure may change, but whether it changes depends on the size of x.

 

Through research, it is found that when x>0.5, the structure of Li1-xCoO2 is extremely unstable, and the crystal collapse will occur, which is manifested externally as the collapse of the battery cell. Therefore, the x value in Li1-xCoO2 should be controlled by limiting the charging voltage during the use of the battery cell. Generally, if the charging voltage is not greater than 4.2V, then x is less than 0.5, and the crystal form of Li1-xCoO2 is still stable.

 

The negative electrode C6 itself has its own characteristics. After the first formation, the Li in the positive electrode LiCoO2 is charged into the negative electrode C6, and when discharged, Li returns to the positive electrode LiCoO2. However, after the formation, a part of Li must remain in the center of the negative electrode C6 to ensure the normal embedding of Li in the next charge and discharge, otherwise the collapse of the battery cell is very short. In order to ensure that a part of Li remains in the negative electrode C6, it is generally achieved by limiting the lower limit voltage of discharge: the upper limit voltage of safe charging is ≤4.2V, and the lower limit voltage of discharge is ≥2.5V.

 

The principle of memory effect is crystallization, which almost never occurs in lithium batteries. However, the capacity of lithium batteries will still decrease after multiple charging and discharging, and the reasons are complex and diverse. The most important thing is the change of the positive and negative electrode materials themselves. From a molecular level, the hole structure that accommodates lithium ions on the positive and negative electrodes will gradually collapse and block; from a chemical point of view, it is the active passivation of the positive and negative electrode materials, and side reactions will occur to generate other stable compounds. Physically, there will also be gradual peeling of the positive electrode material, which will eventually reduce the number of lithium ions that can move freely in the battery during charging and discharging.

 

Overcharging and overdischarging will cause permanent damage to the positive and negative electrodes of lithium batteries. From a molecular level, it can be intuitively understood that overdischarging will cause excessive release of lithium ions from the negative electrode carbon, causing its sheet structure to collapse, and overcharging will force too many lithium ions into the negative electrode carbon structure, making some of the lithium ions no longer released.

 

Unsuitable temperature will trigger other chemical reactions inside the 1.5v Alkaline battery to generate compounds that we do not want to see, so a protective temperature-controlled diaphragm or electrolyte additive is set between the positive and negative electrodes of many lithium batteries. When the battery temperature rises to a certain level, the pores of the composite membrane close or the electrolyte denatures, the internal resistance of the battery increases until the circuit is broken, and the battery no longer heats up, ensuring that the battery charging temperature is normal.

 

2. 1.5v Alkaline battery formula and process flow

 

1. Positive and negative electrode formula

 

1.1 Positive electrode formula: LiCoO2+conductive agent+adhesive+current collector (aluminum foil)

 

LiCoO2 (10μm): 96.0%

 

Conductive agent (CarbonECP) 2.0%

 

Adhesive (PVDF761) 2.0%

 

NMP (newly added adhesiveness): the weight ratio of solid matter is about 810:1496

 

a) The viscosity of the positive electrode is controlled at 6000cps (temperature 25 rotor 3);

 

b) The weight of NMP must be appropriately adjusted to meet the viscosity requirements;

 

c) Pay special attention to the effects of temperature and humidity on viscosity

 

Positive electrode active material:

 

Lithium cobalt oxide: positive electrode active material, lithium ion source, to increase the lithium source for the battery. Non-polar substance, irregular shape, particle size D50 is generally 6-8μm, water content ≤0.2%, usually alkaline, pH value is about 10-11.

 

Lithium manganate: non-polar substance, irregular shape, particle size D50 is generally 5-7μm, water content ≤0.2%, usually weakly alkaline, pH value is about 8.

 

Conductive agent: chain-like substance, water content <1%, particle size is generally 1-5μm. Superconducting carbon black with excellent conductivity is usually used, such as Ketjen Carbon ECP and ECP600JD. Its purpose: to improve the conductivity of positive electrode materials and compensate for the electronic conductivity of positive electrode active materials; to increase the liquid absorption of the electrolyte of the positive electrode sheet, add a new reaction interface, and reduce polarization.

 

PVDF binder: non-polar substance, chain-like substance, molecular weight ranges from 300,000 to 3,000,000; molecular weight decreases after water absorption, and viscosity becomes worse. Used to bond lithium cobalt oxide, conductive agent and aluminum foil or aluminum mesh together. Common brands include Kynar761.

 

NMP: a weakly polar liquid used to dissolve/swell PVDF and dilute the slurry.

 

Current collector (positive lead): made of aluminum foil or aluminum tape.

 

1.2 Negative electrode formula: graphite + conductive agent + thickener (CMC) + binder (SBR) + current collector (copper foil)

 

Negative electrode material (graphite): 94.5%

 

Conductive agent (CarbonECP): 1.0% (Ketjen superconducting carbon black)

 

Binder (SBR): 2.25% (SBR = styrene-butadiene rubber latex)

 

Thickener (CMC): 2.25% (CMC = sodium carboxymethyl cellulose)

 

The weight ratio of water: solid matter is 1600:1417.5

 

a) Negative electrode viscosity control 5000-6000cps (temperature 25 rotor 3)

 

b) The weight of water should be adjusted appropriately to meet the viscosity requirements;

 

c) Pay special attention to the effect of temperature and humidity on viscosity

 

2. Positive and negative mixing

 

Graphite: Negative active material, an important substance that constitutes the negative electrode reaction; it is mainly divided into two categories: natural graphite and artificial graphite. Non-polar substances are easily contaminated by non-polar substances and easily dispersed in non-polar substances; they are not easy to absorb water and are not easy to disperse in water. Contaminated graphite is easy to re-agglomerate after being dispersed in water. The general particle size D50 is about 20μm. The particle shapes are diverse and mostly irregular, and the important ones are spherical, flaky, fibrous, etc.

 

Conductive agent: Its uses are:

 

a) Improve the conductivity of the negative electrode sheet and compensate for the electronic conductivity of the negative electrode active material.

 

b) Improve the reaction depth and utilization rate.

 

c) Prevent the appearance of dendrites.

 

d) Use the liquid absorption capacity of conductive materials to improve the reaction interface and reduce polarization. (You can choose to add or not according to the graphite particle size distribution).

 

Additives: Reduce irreversible reactions, improve adhesion, increase slurry viscosity, and prevent slurry precipitation.

 

Thickener/anti-precipitation agent (CMC): a polymer compound that is easily soluble in water and polar solvents.

 

Isopropyl alcohol: a weak polar substance, which can reduce the polarity of the binder solution after addition and improve the compatibility of graphite and the binder solution; it has a strong defoaming purpose; it is easy to catalyze the cross-linking of the binder network and improve the bonding strength.

 

Ethanol: a weak polar substance, which can reduce the polarity of the binder solution after addition and improve the compatibility of graphite and the binder solution; it has a strong defoaming purpose; it is easy to catalyze the cross-linking of the binder line and improve the bonding strength (the uses of isopropyl alcohol and ethanol are essentially the same. When mass-producing, you can consider the cost factor and choose which one to add).

 

Water-based binder (SBR): Bond graphite, conductive agent, additives and copper foil or copper mesh together. Small molecule linear chain emulsion, very soluble in water and polar solvents.

 

Deionized water (or distilled water): diluent, added in moderation to change the fluidity of the slurry.

 

Negative lead: made of copper foil or nickel strip.

 

2.1 Positive electrode mixture:

 

2.1.1 Pretreatment of raw materials

 

1) Lithium cobalt oxide: dehydration. Generally, it is baked at 120°C and normal pressure for about 2 hours.

 

2) Conductive agent: dehydration. Generally, it is baked at 200°C and normal pressure for about 2 hours.

 

3) Binder: dehydration. Generally, it is baked at 120-140°C and normal pressure for about 2 hours. The baking temperature depends on the molecular weight.

 

4) NMP: dehydration. Use dry molecular sieve for dehydration or use special material taking facilities and use it directly.

 

2.1.2 Material ball milling:

 

1) After 4 hours, sieve and separate the ball milling;

 

2) Pour LiCoO2 and CarbonECP into the material barrel, add grinding balls (dry material: grinding balls = 1:1), and ball mill on the roller bottle, and control the speed above 60rmp

 

2.1.3 Raw material blending:

 

1) Dissolution of the binder (according to the standard concentration) and heat treatment.

 

2) Ball milling of lithium cobalt oxide and conductive agent: make the powder initially mixed, and lithium cobalt oxide and conductive agent bond together to improve the conductivity of the agglomeration purpose. After being prepared into slurry, it will not be distributed in the binder alone. The ball milling time is generally about 2 hours. To prevent impurities from being mixed in, agate balls are usually used as ball milling mesons.

 

2.1.4 Dispersion and wetting of dry powder:

 

Principle: Solid powder is placed in the air. As time goes by, it will adsorb part of the air on the surface of the solid. After the liquid binder is added, the liquid and gas begin to compete for the solid surface; if the adsorption force of the solid and the gas is stronger than the adsorption force of the liquid, the liquid cannot wet the solid; if the adsorption force of the solid and the liquid is stronger than the adsorption force of the gas, the liquid can wet the solid and squeeze out the gas.

 

When the wetting angle is ≤90°, the solid is wetted. When the wetting angle is >90°, the solid is not wetted.

 

All members in the positive electrode material can be wetted by the binder solution, so the dispersion of the positive electrode powder is relatively easy.

 

The influence of dispersion method on dispersion:

 

1) Static method (long time, poor effect, but does not damage the original structure of the material);

 

2) Stirring method: rotation or rotation plus revolution (short time, good effect, but may damage the self-structure of individual materials).

 

The influence of stirring paddle on dispersion speed: stirring paddles generally include serpentine, butterfly, spherical, paddle, gear and so on. Generally, serpentine, butterfly and paddle stirring paddles are used to deal with the initial stage of materials or ingredients that are difficult to disperse; spherical and gear-shaped stirring paddles are used for the state of low dispersion difficulty, and the effect is better.

 

The influence of stirring speed on dispersion speed. Generally speaking, the higher the stirring speed, the faster the dispersion speed, but the greater the damage to the material structure and the equipment.

 

The influence of concentration on dispersion speed. Generally speaking, the lower the slurry concentration, the faster the dispersion speed, but too thin will lead to material waste and increased slurry precipitation.

 

The influence of concentration on bonding strength. The greater the concentration, the greater the flexibility and bonding strength; the lower the concentration, the smaller the bonding strength.

 

The influence of vacuum on dispersion speed. High vacuum is conducive to the discharge of gas from the gaps and surfaces of the material, reducing the difficulty of liquid adsorption; the difficulty of uniform dispersion of the material will be greatly reduced when the material is completely weightless or the gravity is reduced.

 

The influence of temperature on dispersion speed. At a suitable temperature, the slurry has good fluidity and is easy to disperse. Too hot slurry is easy to form a skin, and too cold slurry will greatly reduce its fluidity.

 

Dilution: Adjust the slurry to a suitable concentration for easy coating.

 

2.1.5 Operation steps

 

a) Pour NMP into the power mixer (100L) to 80°C, weigh PVDF and add it, and turn on the machine; parameter setting: speed 25±2 rpm, stirring for 115-125 minutes;

 

b) Turn on the cooling system, add the ground positive electrode dry material in four equal times, each time with an interval of 28-32 minutes. The third addition depends on the material to add NMP, and the fourth addition is followed by NMP; power mixer parameter setting: speed 20±2 rpm

 

c) 30±2 minutes after the fourth addition, high-speed stirring is performed for 480±10 minutes; power mixer parameter setting: revolution 30±2 rpm, autorotation 25±2 rpm;

 

d) Vacuum mixing: Connect the power mixer to vacuum, maintain the vacuum degree at -0.09Mpa, and stir for 30±2 minutes; Power mixer parameter settings: revolution 10±2 minutes, autorotation 8±2 rpm

 

e) Take 250-300 ml of slurry and use a viscometer to measure the viscosity; test conditions: rotor number 5, speed 12 or 30rpm, temperature range 25°C;

 

f) Take the positive electrode material out of the power mixer for colloid milling and sieving, and put a label on the stainless steel basin. After handing over to the slurry pulling equipment operator, it can flow into the slurry pulling operation process.

 

2.1.6 Precautions

 

a) After completion, clean the machine equipment and working environment;

 

b) When operating the machine, pay attention to safety to prevent head injuries.

 

2.2 Negative electrode mixing

 

2.2.1 Pretreatment of raw materials:

 

1) Graphite:

 

A. Mix to homogenize the raw materials and improve consistency.

 

B. Bake at 300~400°C under normal pressure to remove surface oily substances, improve compatibility with water-based adhesives, and round the edges and corners of the graphite surface (some materials are not allowed to be baked to maintain surface characteristics, otherwise the performance will be reduced).

 

2) Water-based adhesive: dilute appropriately to improve dispersion ability.

 

2.2.2 Blending, wetting and dispersion:

 

1) Graphite and adhesive solutions have different polarities and are not easy to disperse.

 

2) Graphite can be preliminarily wetted with an alcohol-water solution and then mixed with the adhesive solution.

 

3) The stirring concentration should be appropriately reduced to improve dispersibility.

 

4) The dispersion process is to reduce the distance between polar and non-polar substances and increase potential energy or surface energy, so it is an endothermic reaction, and the overall temperature decreases during stirring. If conditions permit, the stirring temperature should be appropriately increased to make heat absorption easier, while improving fluidity and reducing the difficulty of dispersion.

 

5) If the stirring process is added with a vacuum degassing process, the gas is removed and the solid-liquid adsorption is promoted, the effect will be better.

 

6) The dispersion principle and dispersion method are the same as those in the positive electrode ingredients

 

2.2.3 Dilution:

 

Adjust the slurry to a suitable concentration for easy coating.

 

2.2.4 Material ball milling

 

1) Pour the negative electrode and KetjenblackECP into the material barrel and add the ball mill (dry material: grinding ball = 1:1.2) and ball mill on the roller bottle, and control the speed above 60rmp;

 

2) After 4 hours, sieve and separate the ball mill;

 

2.2.5 Operation steps

 

1) Heat the pure water to 80°C and pour it into the power mixer (2L)

 

2) Add CMC and stir for 60±2 minutes; Power mixer parameter setting: revolution is 25±2 minutes, rotation is 15±2 rpm;

 

3) Add SBR and deionized water and stir for 60±2 minutes;

 

Power mixer parameter setting: revolution is 30±2 minutes, rotation is 20±2 rpm;

 

4) Add the negative electrode dry material in four equal times, and add pure water while adding the material, with an interval of 28-32 minutes each time; Power mixer parameter setting : Revolution is 20±2 rpm, rotation is 15±2 rpm;

 

5) After the fourth feeding, high-speed stirring is performed for 480±10 minutes after 30±2 minutes;

 

Power mixer parameter setting: Revolution is 30±2 rpm, rotation is 25±2 rpm;

 

6) Vacuum mixing: Connect the power mixer to vacuum, maintain the vacuum degree at -0.09 to 0.10Mpa, and stir for 30±2 minutes;

 

Power mixer parameter setting: Revolution is 10±2 minutes, rotation is 8±2 rpm

 

7) Take 500 ml of slurry and measure the viscosity using a viscometer;

 

Test conditions: rotor number 5, speed 30rpm, temperature range 25°C;

 

8) Take the negative electrode material out of the power mixer for grinding and sieving, and label it on the stainless steel basin. After handing over to the slurry pulling equipment operator, it can flow into the slurry pulling operation process.

 

2.2.6 Precautions

 

1) After completion, clean the machine equipment and working environment;

 

2) When operating the machine, pay attention to safety to prevent head injuries.

 

Precautions for batching:

 

Prevent mixing with other impurities;

 

Prevent slurry from splashing;

 

The concentration (solid content) of the slurry should be gradually adjusted from high to low to avoid adding trouble;

 

Pay attention to scraping the edges and bottom during the intermittent mixing process to ensure uniform dispersion;

 

The slurry should not be left for a long time to avoid precipitation or reduced uniformity;

 

The materials to be baked must be sealed and cooled before they can be added to avoid changes in the properties of the component materials;

 

The length of the mixing time is mainly based on the performance of the equipment and the amount of material added;

 

The use of the stirring paddle is replaced according to the difficulty of slurry dispersion. If it cannot be replaced, the speed can be adjusted from slow to fast to avoid damage to the equipment;

 

Before discharging, sieve the slurry to remove large particles to prevent belt breakage during coating;

 

Strengthen training for batching personnel to ensure that they have professional knowledge to avoid major disasters;

 

The key to batching is uniform dispersion. Master this center and other methods can be adjusted by yourself.

 

1. Parameters required for battery production

 

1. Pole piece size

 

2. Slurry drawing process

 

a) Current collector size

 

Positive electrode (aluminum foil), intermittent coating

 

Negative electrode (copper foil), intermittent coating

 

b) Slurry drawing weight requirements

 

3. After positive electrode slurry drawing, the following processes are carried out:

 

Cut large pieces, cut small pieces, weigh pieces (matching pieces), bake, roll pieces, and weld pole lugs. After negative electrode slurry drawing, the following processes are carried out:

 

Cut large pieces, cut small pieces, weigh pieces (matching pieces), bake, roll pieces, and weld pole lugs.

 

4. Rolling requirements

 

5. Matching method

 

6. Pole piece baking

 

Note: The vacuum degree of the vacuum system is -0.095-0.10Mpa; the protective gas is high-purity nitrogen, and the gas pressure is greater than 0.5Mpa.

 

7. Pole lug production

 

a) Positive electrode:

 

The positive pole lug is ultrasonically welded at the positive electrode sheet. The end of the aluminum strip is flush with the edge of the pole sheet.

 

b) Negative electrode:

 

Nickel strip size: 0.10×3.0×48mm, the nickel strip is directly spot welded with a spot welder, and the number of spot welds is required to be 8. The right side of the nickel strip is aligned with the right side of the negative electrode sheet, and the end of the nickel strip is flush with the edge of the electrode sheet.

 

8 Diaphragm size: 0.025×44.0×790±5mm

 

9 Winding needle width: 22.65±0.05mm

 

10 Pressing the core: After the battery is wound, first stick a 24mm wide transparent tape on the bottom of the battery cell, and then use a flattening machine to cold press twice.

 

11 Requirements before the battery cell is put into the shell

 

Tape 1: 10.0×38.0±1.0mm, the tape is evenly distributed on both sides of the battery cell;

 

Tape 2: 10.0×38.0±1.0mm, the nickel strip is in the center of the tape;

 

Tape 3: 24.0×30.0±2.0mm, the tape is evenly distributed on both sides of the battery cell;

 

The right side of the nickel strip is 7.0±1.0mm away from the right side of the battery cell.

 

12 Shelling

 

When shelling, use both hands to apply force at the same time, slowly put the battery cell into the battery shell, and do not scratch the battery cell.

 

13 Negative pole ear welding

 

The negative nickel strip and the steel shell are welded with a spot welder. The welding strength must be guaranteed and false welding is prohibited.

 

14 Laser welding

 

When laser welding, the fixture should be carefully installed. The battery shell and the upper cover can be welded only after they are well matched. Be careful to prevent welding deviation.

 

15 Battery vacuum baking

 

Remarks:

 

(1) The vacuum degree of the vacuum system is -0.095~-0.10MPa;

 

(2) The protective gas is high-purity nitrogen, and the gas pressure is >0.5MPa;

 

(3) Vacuum and inject nitrogen once every hour

 

16 Liquid injection volume: 2.9±0.1g

 

Relative humidity of the liquid injection room: ≤30%, temperature: 20±5℃ Sealing tape: 6mm wide red tape. When sticking the tape, be sure to wipe off the electrolyte at the liquid injection port. Use 2 rubber bands to fix the cotton at the liquid injection port.

 

17 Formation system

 

(1) Opening formation process

 

a) Constant current charging: 40mA×4h; 80mA×6h

 

Voltage limit: 4.00V

 

b) Full voltage inspection, batteries with voltage ≥3.90V are sealed, batteries with voltage <3.90V are sealed with 60mA constant current to 3.90~4.00V, and then steel balls are punched

 

c) Battery cleaning, cleaning agent is acetic acid + alcohol

 

(2) Continued formation system

 

Continued formation according to the procedure:

 

a) Constant current charging (400mA , 4.20V, 10min)

 

b) Sleep (2min)

 

c) Constant current charging (400mA, 4.20V, 100min)

 

d) Constant voltage charging (4.20V, 20mA, 150min)

 

e) Sleep (30min)

 

f) Constant current discharge (750mA, 2.75V, 80min)

 

g) Sleep (30min)

 

h) Constant current charging (750mA, 3.80V, 90min)

 

j) Constant voltage charging (3.80V, 20mA, 150min)

 

(3) Capacity testing

 

Batteries are divided into capacities according to the following levels:

 

After the battery is removed from the cabinet, the voltage is fully tested. Batteries with voltage <3.77V are recharged using a program:

 

(1) Constant current charging (750mA, 3.80V, 10min)

 

(2) Sleep (2min)

 

(3) Constant current charging (750mA, 3.80V, 30min)

 

(4) Constant voltage charging (3.80V, 20mA, 60min)

 

18 Battery re-inspection

 

After the battery is unloaded from the cabinet and divided into different capacities, it is placed at room temperature for 20 days for re-inspection. The steps are as follows:

 

a) Use a shaping machine to shape the battery;

 

b) Fully inspect the battery thickness, voltage, and internal resistance. The classification method is as follows:

 

II. Battery manufacturing process

 

1. (Positive and negative electrodes) dry mixing → wet mixing → roll-coating the paste on the conductive substrate → 3-step drying → winding → trimming (cut into a certain width) → rolling → winding (standby) Dry mixing uses ball milling, and the grinding balls are glass balls or zirconia ceramic balls;

 

Wet mixing uses a planetary powder mixer, whose blades are installed on 2-3 shafts respectively, for better mixing effect. The amount of solvent in wet mixing should be appropriate to form a suitable rheological state to obtain a smooth coating. The electrode paste should be rolled to ensure a certain viscosity. The paste is applied to both sides of the aluminum foil or copper foil, and the thickness of the coating depends on the battery model. Then it passes through three heating zones for drying. NMP (or water) evaporates from the coating with the flow of hot air or dry nitrogen, and the solvent can be recycled. Rolling is to increase the density of the coating and make the electrode thickness meet the size of the battery assembly. The pressure in the rolling stage should be moderate to prevent the powder from scattering during winding.

 

2. Battery assembly

 

Assembly process of cylindrical battery: insulating bottom ring into cylinder → winding battery core into cylinder → inserting mandrel → welding negative electrode collector to steel cylinder → inserting insulating ring → steel cylinder rolling → vacuum drying → liquid injection → welding combination cap (PTC element, etc.) to positive electrode lead → sealing → X-ray inspection → numbering → formation → circulation → aging.

 

The assembly process of square batteries: the insulating bottom is put into the steel box → the sheet-shaped combined battery core is put into the barrel → the negative electrode current collector is welded to the steel box → the upper sealing gasket → the positive electrode current collector is welded to the rod lead → the combined cover (PTC element, etc.) is welded to the rotating lead → the combined cover is positioned → laser welding → vacuum drying → liquid injection → sealing → X-ray inspection → numbering → formation → circulation → aging.

 

Assembly process description: Take the cylindrical battery as an example (the basic process of the square battery is the same). Before the winding core is put into the barrel, the aluminum strip (0.08-0.15mm thick, 3mm wide) and the nickel strip (0.04-0.10mm thick, 3mm wide) are ultrasonically welded to the designated positions of the positive and negative conductive substrates as the current collectors.

 

Battery separators are generally composed of PE/PP2 layers or PP/PE/PP3 layers. The separators are all heat-treated at 120℃ to increase their barrier properties and improve their safety.

 

The positive electrode, separator and negative electrode are stacked and wound into a cylinder. Since the paste electrode is used, the paste material and the matrix must be well combined to form a high-density electrode. In particular, powder must be prevented from falling off to prevent it from penetrating the separator and causing an internal short circuit in the battery.

 

Before the wound battery cell is inserted into the steel cylinder, an insulating bottom is placed at the bottom of the steel cylinder to prevent internal short circuits in the battery. This is the same for general batteries.

 

The electrolyte generally uses LiPF6 and non-aqueous organic solvents. Before vacuum injection, the battery must be vacuum dried for 24/h to remove moisture and moisture from the battery components to prevent LiPF6 from reacting with water to form HF and shortening the life.

 

The battery is sealed by applying sealant, inserting gaskets, crimping and cross-sectional shrinkage. The basic principle is the same as that of alkaline rechargeable batteries. After sealing, the battery should be cleaned of oil and splashed electrolyte with a mixture of isopropyl alcohol and water, and then dried. Use an odor sensor or "sniffer" element to check battery leakage.

 

After the entire battery is assembled, the battery is X-rayed to identify whether the internal structure of the battery is normal, and to check whether the battery core is not correct, the steel shell cracks, the welding point conditions, and whether there is a short circuit, etc., to exclude batteries with the above defects to ensure the quality of the battery.

 

The last process is formation. When the battery is charged for the first time, a protective film is formed on the anode, called the solid electrolyte interphase layer (SEI), which can prevent the anode and electrolyte from reacting and is a key factor in the safe operation, high capacity, and long life of the battery. After several charge and discharge cycles, the battery is aged for 2-3 weeks, the micro-short-circuited batteries are removed, and then the capacity is sorted and packaged to become a commodity.

 

III. Battery performance

 

1. Electrical performance:

 

(1) Rated capacity: 0.5C discharge, the discharge time of a single cell is not less than 2h, and the discharge time of a battery pack is not less than 1h54min (95%);

 

(2) 1C discharge capacity: 1C discharge, the discharge time of a single cell is not less than 57min (95%), and the discharge time of a battery pack is not less than 54min (90%);

 

(3) Low temperature discharge capacity: 0.5C discharge at -20°C, the discharge time of a single cell or a battery pack is not less than 1h12min (60%);

 

(4) High temperature discharge capacity: 0.5C discharge at 55°C, The discharge time of a single cell shall not be less than 1h54min (95%), and the discharge time of a battery pack shall not be less than 1h48min (90%);

 

(5) Charge retention and recovery capability: when fully charged and stored at room temperature for 28 days, the charge retention discharge time shall not be less than 1h36min (80%), and the charge recovery discharge time shall not be less than 1h48min (90%);

 

(6) Storage performance: the single cell or battery pack for storage test shall be selected from those with a production date of less than 3 months, and shall be charged to 50% to 60% of the capacity before storage, and stored for 90 days in an environment with an ambient temperature of 40℃±5°C and a relative humidity of 45% to 75%. After the storage period expires, remove the battery pack, fully charge it with 0.2C and leave it for 1 hour, then discharge it at 0.5C constant current to the termination voltage. The above test can be repeated 3 times, and the discharge time shall not be less than 1h12min (60%);

 

(7) Cycle life: The battery or battery pack is charged with 0.2C and discharged with 0.5C for cycles. When the discharge capacity is less than 72min (60%) for two consecutive times, stop the test. The cycle life of a single cell shall not be less than 600 times, and the cycle life of a battery pack shall not be less than 500 times;

 

(8) High temperature shelf life: Single cells with a production date of less than three months shall be selected for high temperature shelf life test. Before shelf life, they shall be charged with 50%±5% of the capacity and then left at an ambient temperature of 55℃±2°C for 7 days. After 7 days, remove the battery and leave it at an ambient temperature of 20℃±5°C for 2~5h. First discharge the battery to the termination voltage at 0.5C, then charge it at 0.2C after 0.5h, let it stand for 0.5h, and then discharge it to the termination voltage at 0.5C constant current. This capacity is used as the recovery capacity. The above steps are a one-week cycle, and the test ends when the discharge time in a certain week is less than 72min (60%). The shelf life is not less than 56 days (8-week cycle).

 

2. Safety performance

 

(1) Continuous charging: Charge the single cell at a constant current of 0.2ItA. When the terminal voltage of the single cell reaches the charging limit voltage, change to constant voltage charging and maintain it for 28d. After the test, there should be no leakage, deflation, rupture, fire, or explosion (equivalent to full-charge floating charge).

 

(2) Overcharge: Charge the single cell with a constant current and voltage regulator at 3C, and switch to constant voltage charging after the voltage reaches 10V, until the battery explodes or catches fire or the charging time is 90min or the battery surface temperature is stable (temperature difference within 45min ≤2°C), stop charging, the battery should not catch fire or explode (3C10V); Charge the battery pack with a voltage regulator at 0.5ItA constant current, and switch to constant voltage charging after the voltage reaches n×5V (n is the number of single cells in series), until the battery pack explodes or catches fire or the charging time is 90min or the battery pack surface temperature is stable (temperature difference within 45min ≤2°C), stop charging, the battery should not catch fire or explode.

 

(3) Forced discharge (reverse charge): First discharge the single cell at a constant current of 0.2ItA to the termination voltage, then reverse charge the battery at a current of 1ItA. The charging time is required to be no less than 90min. The battery should not catch fire or explode. Discharge one of the single cells in the battery pack to the termination voltage, and the rest are fully charged batteries. Then discharge at a constant current of 1ItA until the voltage of the battery pack reaches 0V. Stop discharging. The battery should not catch fire or explode.

 

(4) Short circuit test: Short-circuit the single cell externally.

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